Cross-polarized dual feed



sept. 27, 1960 R. F. H. YANG 2,954,557

CROSS-POLARIZED DUAL FEED Filed June 15, 1959 FIE. 5

United States Patent CROSS-POLARIZED DUAL FEED Richard F. H. Yang,Orland Park, lll., assignor to Andrew Corporation, Cook County, lll., acorporation of Illinois Filed June 15, 1959, Ser. No. 820,370

7 Claims. (Cl. 343-756) This invention relates to microwave propagationdevices, and more specifically lto a cross-polarized dual feed forparabolic antennas.

In a co-pending application of the same inventor, Serial No. 615,199,led October l0, 1956, there is described and claimed an improved type ofcross-polarized feed. Various embodiments of the novel feed constructionare described in that application. The feed construction thereinillustrated employs a horn cavity which surrounds the end of a triaxialtransmission line, the electromagnetically open end of the horn facingback along the line, the end of the triaxial line within the cavityhaving means for exciting or energizing the cavity by the signals in thetwo transmission paths of the triaxial line in cross-polarized relation.The horn cavity is excited from one of the transmission paths at theanterior (open) end of the horn, and is excited from the othertransmission path of the triaxial line at the posterior end, theintermediate portion having radial conducting members extending betweenthe transmission line and the wall of the cavity in the directionparallel with the electric eld vector of the anteriorly energized signalto prevent this signal from entering the posterior portion of the cavityand then again being reflected forward, the isolation of the anteriorfrom the posterior portion for signals of the polarization of theanterior signal thus preventing radiation from the horn of anteriorsignals which have travelled any substantial distance in the horn. It isWell known that preservation of exact direction of polarization of anygiven mode becomes more difficult with length of the path traversed by asignal in a waveguide or other cavity (of which a horn is merely aspecial case). The various embodiments illustrated in the co-pendingapplication, by cutting olf the reflected path of the anterior signal,are accordingly highly effective Iin aiding in preservation of theperpendicular relation between the polarizations of the transmissionsexcited in the anterior and posterior portions of the horn.

It is the principal object of the present invention to further improvethe type of feed shown in the co-pending application by providing astructure in which the posteriorly excited signal is also brought to themouth of the horn in its exact intended polarization, and to provide asimple manner of accomplishing this object with a minimum of effect onthe anteriorly energized signal.

ln the attainment of this principal object, there has been developed anovel type of mode-transition construction which, although being ofpeculiar utility in crosspolarized dual feeds of the general type shownin the copending application, will also be found to be of more generalutility in launching transmissions from or to concentric lines,particularly where exactness of direction of polarization is requiredand where the concentric line impedance is to be matched for radiationin space and similar purposes.

For understanding of the invention, and the manner in which the objectsstated above are accomplished, and other advantages of the inventionobtained, reference is lee made to the embodiment of the inventionillustrated in the annexed drawing, in which:

Figure 1 is 'a side view, partially broken away in section, of a rdualfeed made in accordance 'with the teachings of the invention;

Figure 2 is a View taken along the line 2 2 of Figure l;

Figure 3 is a transverse sectional view taken along the line 3 3 ofFigure 2;

Figure 4 is` a transverse sectional View taken along the line 4 4 ofFigure 2; and

Figure 5 is a transverse sectional View taken along the line 5 5 ofFigure 2.

The feed illustrated lin the drawing employs a horn cavity 10 ofcircular cross-section, having a closure 12 on the posterior end and aflange 14 on the anterior end, to which is suitably secured a dielectricwindow 16, thus forming a cup-shaped cavity which is electromagneticallyopen at one end. In the illustrated embodiment, the horn 10 is` ofuniform diameter throughout its length, since the illustrated feed isdesigned for uniform illumination of a parabola from a short distance,the feed being supported on the axis of the parabola by a triaxial line;employment of the illustrated structure in a flared horn, circular ornon-circular, will, however, be obvious.

The horn cavity 10 contains diametrically opposed fins or ridges 18 and20 which are relatively wide at the posterior end of the cavity and arecontinuously tapered so that the gap between them widens to the fulldiameter of the cavity at the extreme anterior end. Entering the cavitycentrally of the window 16 is a triaxial line 22, having `an innerconductor 24 and an intermediate conductor 26 forming, in the yannulustherebetween, an inner transmission path, and an outer conductor 30, thelatter forming with the intermediate conductor 26 an outer transmissionpath 32. The end portion of the triaxial line 22 is Within the cavity10, extending in the gap between the fins to a point just short of theposterior end closure 12. y

In the portion of the outer conductor 30 at the anterior end of thecavity are diametrically opposed longitudinal slots 34, preferablyhalf-wave slot radiators at the frequency of transmission of the outertransmission path. The intermediate conductor 26 terminates in thecentral longitudinal portion of the cavity and is there joined to theouter conductor 30 by an annular conducting end wall 36, thusconstituting the portion of the triaxial line rearward of thistermination of the outer transmission path a coaxial extension of theinner transmission path, employing the innermost conductor 24 and theoutermost conductor 30 as its concentric conductors in this region.

The intermediate conductor 26 is joined by a radial short circuitconnection 38 in a plane perpendicular to the plane formed by the slots34, in a position of longitudinal correspondence with the slots. Dipoleelements 40 extend radially from the outer surface of the outerconductor 30 in the direction perpendicular to the plane of the slots34.

It will be seen that the outer transmission path of the triaxial cableis provided with the means above described for coupling a ysignal in theouter path to the cavity, the slots 34 and the short at 38 cooperatingto convert the radially symmetrical transmission mode of the coaxialline to the TEM propagation mode in the cavity, the central electric eldvector in this mode being aligned along a diameter, the two halves ofthe outer conductor 30 on opposite sides of the plane of the slots beingin' opposite phase. The dipole elements 40, .in alignment with thiselectric field vector, serve to aid in coupling the mode thus energizedto the cavity. It will be observed that the manner of coupling thetransmissions of the outer path to the anterior portion of the cavity,as illus- 3 trated and described, is substantially identical with thatshown in 'some of the embodiments of the invention described and claimedin the co-pending application.

The extreme end portion of the outer conductor 30 has diametricallyopposed quarter-wave slots 42 in the plane perpendicular to the planeformed by the slots 34, and also perpendicular to the plane of the fins18 and 20, which lie in the same plane as that of the slots 34. Theslots 42 divide the end of the outer conductor 30 into portions 44 and46 which are respectively adjacent to, but spaced from, the edges of theiins 18 and 20, the extreme end of the inner transmission path of thetriaxial line 22 thus lying in the narrow portion of the gap between thens. The central conductor 24 is shorted at 48 radially to the endportion 44, thus producing collapse of the electric field in thisregion, and converting the circularly symmetrical coaxial cablepropagation mode to a mode wherein the end portions 44 and 46 are atopposite potential, in a manner similar to that previously described inconnection with the energizing of the anterior portion.

There is thus produced at the posterior end of the cavity, in responseto the signals of the inner transmission path ofthe triaxial line, amode of substantially the same characteristics as that produced in theanterior portion from the outer transmission path, except that in theposterior portion the field pattern is rotated by 90, the ns 18 and 20being coupled by proximity to the oppositely phased end portions 44 and46 of the line.

The TEU mode generated in the posterior portion is coupled to theexterior of the horn by the tapered fins, which serve both to providethe desired impedance transformation for optimum radiation and topreserve the exact direction of polarization of the posteriorly excitedsignal.

It will be noted that the illustrated manner of coupling the posteriorsignal to the cavity and thence to the exterior produces a minimum ofinterference with the preservation of proper polarization of theanteriorly launched signal. The latter signal is prevented from enteringthe posterior portion of the cavity by shorting posts 50 which connectthe outer conductor of the triaxial cable to the walls of the cavity inthe intermediate region between the two mode-transition assemblies,these shorting posts being parallel with the dipole elements 40 and thusin the direction of the diametric electric ield vector of the anteriorlylaunched signal. Thus propagation of the anteriorly launched signal isconfined -to the anterior region of the cavity. In this region of thecavity, the fins 18 and 20 are of substantially reduced width. Inaddition, because of the relative relation of the elements described,the electric field vector of the TEM mode launched anteriorly is, in theregion of the ns 18 and 20, perpendicular to these ns, so that not onlydoes the presence of these fins in the relatively short distance ofpropagation of the anterior signal to the window not change thepolarization of the anterior signal, but in addition the attenuation ofthe anterior signal is extremely small.

It will thus be seen that the illustrated embodiment in all respectsprovides, in a very simple manner, highly desirable characteristics forthe transmission of crosspolarized signals with a minimum of cross-talkand attenuation. It will also be seen, however, that many modifications,some obvious and some apparent only on study, may be made withoutimpairing the advantages obtained by the invention. Additionally, thoseskilled in the art will readily observe that the novel manner ofcoupling the concentric line to the cavity used at the posterior end mayreadily be employed in other types of structures. Accordingly, the scopeof the protection to be given the invention should not be considered tobe limited by the particular embodiments shown, but should extend to thestructures defined in the appended claims, and equivalents thereof.

What is claimed is:

1. A cross-polarized antenna feed comprising a triaxial transmissionline having concentric annular transmission paths each having innermostand outermost conductors, a cavity surrounding the end portion of thetransmission line and having an electromagnetically open end facing backalong the transmission line, diametrically opposed longitudinal slots inthe outermost conductor of the outer transmission path in the anteriorportion of the cavity, diametrically opposed longitudinal slots in theextreme end of the outermost conductor of the inner transmission path ina plane perpendicular to the plane of the first slots, means within therespective transmission paths for producing electric fields across saidrespective pairs of slots, oppositely disposed iin members on the wallsof the cavity in the plane of the first slots, tapering in width fromthe posterior to the anterior portion of the cavity, and opposi-telydisposed radially extending shorting members connecting the wall of thecavity with the transmission line in the longitudinal region between thepairs of slots and in the plane of the second pair of slots.

2. An antenna feed horn comprising a coaxial transmission line, yacavity surrounding the end of the transmission line and having anelectromagnetically open end facing back along the line, oppositelydisposed iin members on the wall of the cavity tapering in width fromthe posterior to the anterior portion of the cavity, the end of the'transmission line beingr in the posterior portion of the cavity betweenthe iin members, and mode transition means on the end of thetransmission line to produce an electric field between the portions ofthe outer conductor adjacent yto respective fin members.

3. A microwave mode-transition assembly comprising opposed tapered nsforming a tapered gap having wide and narrow longitudinal portions, acoaxial transmission line having the end thereof in the narrow portionof the gap closely adjacent to, but spaced from, the fins and extendingmidway in the gap from said end toward the wide portion of the gap, andmode-conversion means on said end of the transmission line fortransforming the coaxial-cable mode to a mode having an electric fieldin the direction crosswise of the gap.

4. The assembly of claim 3 wherein the mode-conversion means compriseslongitudinal quarter-wave slots in the end of the outer conductor of thetransmission line in a plane perpendicular to the plane of the tins anda radially extending short-circuit extending from the inner to the outerconductor in the plane of the fins.

5. A cross-polarized dual mode-transition assembly comprising theassembly of claim 3 wherein the coaxial line comprises one of thetransmission paths of a triaxial line, and there are provided means onthe end portion of the line to couple the other transmission path to theexterior in a mode having an electric field perpendicular to the gap.

6. A cross-polarized feed for parabolic antennas comprising themode-transition assembly of claim 5 having a cavity surrounding the endportion of the triaxial line and electromagnetically open at the endfacing back along the line, the fins being on opposite walls of thecavity.

7. The cross-polarized feed of claim 6 having the coupling means forsaid other transmission path in the anterior portion of the cavity, thenarrow portion of the gap being in the posterior portion, and havingradial shorting members in the intermediate portion of the cavity in theplane perpendicular to the gap to isolate the two coupling means andprovide support for the transmission line.

No references cited.

